Some U.S. government media collections are licenses specifically as government works. See https://www.usa.gov/government-copyright
Decorative images used to liven up a presentation or other project may not need to be specifically health sciences related. The sources that follow are good options for decorative images. Copyright applies here too. So be sure to know whether the images are copyright protected, Creative Commons licenses, or in the public domain. Additionally, see the Privacy & Publicity Rights section of this guide.
Read their terms of use for more information about how icons and photos can be used.
Filtering instructions:
These images are NOT in the public domain nor Creative Commons licensed. These sites provide images that are free to use under their own licensing agreements.
Read the Morguefile license to learn how images can be used, and read the full license for more information.
Read the Pixabay license to learn the full set of terms for using Pixabay images.
Read the Unsplash terms to learn how their images can be used.